For secondary batteries, shortening a charging time is an important element for improving convenience for users. A method of charging at night for several hours as in the conventional art precludes equipment that uses the secondary battery from being used the next day if the user forgets to do the charging at night. Similarly, the method of charging for several hours needs a secondary battery of sufficient capacity to store daily electricity, leading to an increased volume of the secondary battery. On the other hand, secondary batteries that can be rapidly charged can be charged immediately before use.
For secondary batteries used for electric cars, shortening a charging time is also an important element for improving convenience for users. Enabling the charging time to be shortened allows the capacity of the secondary battery mounted in the electric car to be reduced in areas where charging stations are installed at regular intervals. Moreover, a delay in infrastructure development for charging stations has been a challenge to electric cars, and enabling the charging time to be shortened reduces a waiting time for charging, promoting installation of charging stations.